High Poly Mesh UV Mapping
Since, it is not possible to unwrap a highpoly sculpted mesh (i am thinking about 500k polys) in blender. You are left with few choices.

duplicate your highpoly mesh, add a deciamte modifier, decimate and apply the modifier. Turn your Triangles to Quad and then unwrap it. You can always get Details by baking normals. (Worst idea in my opinon, but fastest).

or

Retopologize your model (best option) , unwrap retopo and bake normals, ao, cavity maps.
(retopo is good too , when you want Hard surface and edges too). 

By the way, There is no point in giving highpoly mesh to your friend for texturing, as it wont be usable in game, but render only.